The invention relates to a method of decontaminating soils such as sediments, silts and muds which include at least a sand fraction and a fine particle fraction wherein the fine particle fraction includes an organic silt component to which particularly contaminants of heavy metals and organic compounds adhere and wherein the contaminated silt fraction is to be separated from the mineral silt fraction.
Contaminated soils are present all over the world in large amounts as contaminated sediments and muds in flowing and still waters and in muds from sewage treatment plants. There are more than 200,000 suspected contaminated areas alone in Germany. Annually, in the area of the former West Germany 60,000 m.sup.3 of contaminated soil are excavated and about 5,000,000 m.sup.3 sewage treatment muds are generated.
These amounts cannot all be deposited. It is therefore absolutely necessary to find methods of utilizing these amounts by new environmentally acceptable methods in order to control these amounts of waste materials in an effective manner. Basically, it would first be necessary to provide economical methods by which the amounts of such contaminated materials being generated is minimized.
A second step of controlling the amount of contaminated materials would be to provide methods for decontaminating or utilizing the contaminated soils remaining after the first control step. But if there is no economical way of utilizing the remaining contaminated materials those materials must be deposited. However, today's deposits are tomorrows contaminations.
Contaminated soils, sediments and muds have been separated in the past by simple hydraulic procedures into sand and fine particle fractions (&lt;63 .mu.m). The fine particle fraction comprises a mineral silt fraction (2 .mu.m to .ltoreq.63 .mu.m) and a clay fraction (&lt;2 .mu.m), and organic contaminants such as heavy metals or heavy metal compounds. The sand fraction is essentially free of contaminations and can generally be utilized otherwise. Since, inspite of many attempts and proposals, economical decontamination processes of contaminated soils do not exist the contaminated silt fractions are generally deposited as waste.
Contaminated silts are included for example in oil-containing anthropogen-affected soils, in sediments and muds of flowing and standing waters (excavation materials) and in industrial wastes, commercial wastes and domestic wastes.
During examination of contaminated river sediments, it was found that the contaminants are concentrated essentially in the fine particle fractions. Many of the known decontamination processes as mentioned above are not suitable if the silt fraction in the soil to be decontaminated is greater than a certain limit such as 30%. It has to be taken into consideration that for example the linear distribution coefficients between sediment and the surrounding water for hexachlorobenzine (HCB) is for example between 2000 and 3000. The distribution coefficient between the carbon fraction in the sediment and the water is even higher that is about 50,000. Hexachlorobenzine consequently has, like almost all organic contaminants, a high affinity to sediments and an extremely high affinity to the carbon fraction attached thereto. Tests for the heavy metal area show that also most of the heavy metals present are preferably associated with the organic fraction.
It has been known so far to oxidize only polycyclic aromatic hydrocarbons contained in synthetic oil-water emulsions utilizing a spray reactor for an oxidative disintegration. However, the disadvantage of such an oxidation medium, such as ozone is that it requires very long oxidation times and a very expensive and large equipment. It has also been tried to remove chlorinated solvents from water by evaporation in a cavitation area, that is, a chemical compound conversion under cavitation conditions. However, subjecting a mixture or a suspension to cavitation has only limited effects since the efficiency of the conversion or, respectively, the release of the contaminants contained therein is limited and, on the way to a subsequent separation apparatus, the component to be separated from the mixture or the suspension may be readsorbed by the fine particles which are contained therein and from which they had just been separated.
It is consequently the object of the present invention to provide a method of decontaminating soils by which mineral fractions of soils, sediments, silts and muds can be effectively separated so that, at the end of the method according to the invention, the remaining volume which cannot be utilized is so small that it is essentially negligible or that it can be easily converted chemically for neutralization while the largest fraction can be utilized without incurring environmental problems. The method should also require only relatively inexpensive equipment and it should be possible to perform the method in a simple and inexpensive way.